This invention relates generally to semiconductor devices, and more particularly the invention relates to dynamic random access memories (DRAM) and cell structures.
Dynamic random access memory (DRAM) is a major semiconductor product and has been the "technology driver" for the semiconductor industry. The commercial success of DRAM results from the smallness of the memory cell unit, which makes high storage density and low cost possible. DRAM cell contains one transistor for controlling READ/WRITE operations and one capacitor for charge storage. The transistor uses the smallest feature size transistor available thus drives the front-end technology such as lithography, transistor scaling and isolation. The technology developed for DRAM can later be used for fabricating other semiconductor products.
However the challenge of DRAM technology of 64 megabits and beyond has been more on how to build a large capacitor in a small area rather than on transistor scaling. The storage capacitor needs about 30 fF to provide enough signal-to-noise ratio. Novel dielectric-constant material or novel memory cell structure become the main focuses for current DRAM technology. Besides the fact that the fabrication process of 64 megabits DRAM and beyond is becoming more and more difficult, the development and manufacturing cost is increasing tremendously yet the technology developed for DRAM is not extendible to other semiconductor products. The limit of C.sub.s =30 fF is not likely to be lessened if the principles of the cell operation, the charge-sharing between the storage capacitor and the READ (bit line) capacitor (i.e., the single-event upset READ scheme), remain unchanged, and will only become more stringent for gigabits DRAM technology.
The concept of "gain cell" has been proposed to alleviate the dependence on large cell capacitance. Many gain cells proposed use an additional transistor (e.g., JFET Complementary MOS, or BJT) to amplify the charge stored in a smaller capacitor whose size is lowerbounded by soft-error consideration (.about.10 fF). However these gain cells either take larger cell layout area or involve complicated fabrication process, both of which increase development and manufacturing costs. Another category of gain cells use the concept of "dynamic threshold." These gain cells do not need large storage capacitors, however, gain cells based on the dynamic threshold operation are sensitive to the processing condition and maybe difficult to manufacture.